126 ANIMAL LIFE 



song rather than any structural difference which the 

 anatomist can detect. In this, as in all other bodily 

 excellences, the higher type of brain is the seat of 

 success. 



Throughout this long series of breathing, heating, 

 and sounding mechanisms we see the conquest of the 

 air, and the inward use of oxygen for effecting move- 

 ment, for maintaining that destructive aspect of the 

 double process of life in virtue of which existence and 

 growth are alone possible. But deeper than any 

 analysis we have yet made are the adaptations of 

 animal structure to ensure its increasingly efficient 

 working in passing from water to land, from cold to 

 warm blooded beings, from a silent to a vocal life. 

 The heart and blood-vessels, even the blood itself, 

 share in the advance, and become in birds and mam- 

 mals divided into two systems, one of which receives 

 aerated blood from the lungs, whilst the other receives 

 the waste from the tissues. From this change all 

 the tissues gain advantage. The muscles become more 

 thoroughly oxidised, and therefore stronger. Their 

 tone reacts on the body generally, and on the nervous 

 system in particular. The heat they exhale becomes 

 more generous, and the more abundant food they 

 require as fuel stimulates more strongly the glands 

 of the body. The trembling balance of life becomes 

 steadied by a firm central nervous control, and its 

 two emulating processes — the downward pull of oxida- 

 tion and the upward thrust of nutrition — are intensified 

 in the maintenance and growth of every part. 



